Work over the past seventeen years has reclassified Blastocystis hominis as a protozoan, removing it from the yeasts where it has languished for seventy years. B. hominis is now firmly established as a member of the Sporozoa, an intestinal parasite and pathogen of man and primates. This protozoan is now a subject of national and international interest to parasitologists and physicians as a cause of diarrhea in man. Treatment is usually with metronidazole. Trimethoprim-sulfamethoxazole is also used successfully. Ultrastructural evidence has proven that B. hominis mitochondria are indeed typical mitochondria. It has been stated that the B. hominis organelle was not a mitochondrion since the tubular cristae appeared to open to the outside; using special techniques, we provided clear photographs of the enveloping outer membrane.